Any tips on how i should change the water? i have some spare containers lying around but they've been washed with soap and left in the cupboard for awhile...i still have the small container that the fish came with so i was thinking i should put it back with some of the tank water for few hours and let the new water settle in the tank.

With a tank that itty bitty, you'll need to do at leat one 100% water change daily. Just pour some of the tank water into a cup/tubberware, put your betta in the tubberware, empty out the tank, scrub it down with your hands with hot water (no soap), fill with clean room temperature water (it can be slightly warmer than room temperature, but not by much), dechlorinate it, scoop up your betta and put him in.

if you have a tubberware or sterlite tub that is bigger than your current tank, use that as your tank. I know it doesn't look pretty at all, but it'll be bigger and safer for your bettas health.

you cant just scoop up your betta and throw him in you have to acclimate your betta to the water so put your betta in the tupperware container then put the container in the tank (with the lid on) and wait about 30 min then put him in his tank